1. Field of the Invention
The subject invention pertains to the field of control circuits, and more particularly to a circuit for sensing the state of a variable state device such as a switch. The invention is particularly applicable to sensing the state of an operator controlled switch of the type that are typically located on a steering wheel of a vehicle, but is also applicable to any environment where hard wire signal communication is particularly undesirable due to structural restrictions or a particularly harsh environment.
2. Description of the Related Art
The continuous increase of convenience features in passenger cars makes the wiring harness and connecting problems thereto a more important issue in terms of safety, reliability, cost and complexity of management. If multiplexing of such communication signals provides a solution in some cases, the vulnerability of this solution in case of bus wiring breakage in areas where numerous functions play a safety role is of significant concern. Examples are safety related switches in the steering wheel/stalk area, or the window lifter switches which are located off doors and control the window motors through the bus. A wireless solution would reduce the risk of wiring breakage, and permit easy relocation by OEMs of switches that do not have a dedicated location (mirrors, seats, windows, etc.) and are remote from the actuators.
Communication of control signals from a steering wheel through the steering column of a vehicle control module has long been a matter of special concern due to the difficulties in maintaining dependable signal communication from a rotating wheel and due to the sever space constraints occurring as a result of the requirement of airbag deployment from the wheel. A detailed discussion of the various hardware techniques, as well as communication through a rotary transformer are disclosed in commonly-assigned U.S. Pat. No. 5,636,863. Ideally, all communication would occur through wireless transmission to avoid the problems of a wiring harness disposal in a rotating wheel, but the relatively electrically noisy environment of an automotive vehicle has generally presented too difficult a setting to provide acceptably reliable communication for control circuitry.
Surface Acoustic Wave (SAW) devices are well known as passive devices that are typically useful for providing an easily recognizable identification signal without need of active electrical power supply to the SAW device itself. The use of a signal communication device that is essentially passive in nature would be particularly advantageous in a steering column application, and such an assembly has already been suggested. However, such suggested applications have not been able to employ such a SAW device in combination with wireless signal communication. Specifically, coupling between a control module and a SAW device has required hard wired signal communication or coupling coils to accommodate relative rotation between the wheel and steering column. Thus, problems still remain in such a system of accommodating coil disposition in the steering column and the signal wires thereof running among operator actuated control switches, the SAW device and the coupling coil.
The present invention contemplates a new and improved wireless switch device and control assembly which overcome the problems of hard wire or coupling coil signal communication while including a SAW device for monitoring switch conditions to provide a new device and circuit assembly which is simple in design, economical to manufacture, readily adaptable to a plurality of uses for control communication or signal monitoring in a wide variety of applications and which provides improved ease of signal communication with high reliability and communication integrity.